Electro-optical apparatus



Nov. 7, W39. w. P. MASON ELECTED-OPTICAL APPARATUS Filed Aug. 11, 1957 FIG i FIG. 2

A 77 OEWE 9" Patented Nov. 7, 1939 UNITED STATES Fries PATENT ELECTED-OPTICAL APPARATUS Warren P. Mason, West Orange, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application August 11, 1937, Serial No. 158,505

18 Claims.

This invention relates to light valves and more particularly to devices of this type including piezoelectric crystals.

While it has heretofore been proposed to utii' lize piezoelectric crystals as light valves in sound recording and for similar purposes where the signal current comprises a continuous band of current variations extending over a very narrow I frequency range, they have not gone into practical use even for these purposes, because the frequency band width, to which the response of a crystal will be substantially uniform when actuated by driving potentials of the same magnitude,

is only a very small fraction of one per cent of the resonant frequency of the crystal.

after described in detail a resistance and an inductance are connected in series with a piezoelectric crystal, the value of the inductance being so chosen that the circuit resonates at two limiting frequencies about equidistant from the anti-resonant frequency of the crystal, whereby the voltage across the crystal is approximately equal to the input voltage at the antiresonant frequency and becomes greater at the two limiting frequencies depending upon the value of the resistance. Since the mechanical resonant frequency of the crystal occurs very nearly at the anti-resonant frequency of the circuit, the voltage required to cause a given amount of strain in the crystal is a minimum at this frequency and increases rapidly on either side thereof. By properly adjusting the value of the series resistance, the amplitude of response of the crystal may be made equal at the three frequencies under consideration, whereby the deformation of the crystal will be substantially uniform over the range identified by the limiting frequencies, the variation of the plane of polarization of light passing through the crystal will be substantially the same and hence the amount of light transmitted therethrough will be substantially cal system and may be expressed as the square root of the value of the series capacity of the equivalent crystal circuit divided by the sum of the values of the series and shunt capacities of this equivalent circuit. However, since the shunt capacity is much greater than the series 'capacity, the coefficient of coupling is substantially equal tothe value of the series capacity divided by that of the shunt capacity.

The series capacity of the crystal being dependent upon the piezoelectric constants of the quartz and other factors which are not readily determined, it is more convenient to express the coefiicient of coupling in terms of the anti-resonant and the resonant frequencies. This coefficient is equal to the square root of twice the difference between the anti-resonant andthe resonant frequencies of the crystal, divided by its anti-resonant frequency.

The invention also contemplates the idea of connecting in shunt to the crystal a variable con-- denser, the capacity of which may be adjusted to vary the frequency band width over which a uni form response may be obtained. The coupling coefiicient and the band width beinga maximum when the series capacity is a maximum, it is preferable to plate the crystal with metal to reduce the air-gap to a minimum and to make contact with the coatings at points which clamp the crystal at the nodes. Again, since secondary modes of vibration over the frequency range applied to the crystal seriously alter its response and may cause the crystal to shatter and the third harmonic longitudinal vibration along the length (Y axis) is free from secondary frequencies when the dimensions of the crystal are kept Within While the apparatus described above may be used for sound recording and picture transmis-f sion, the invention contemplates the use of such apparatus to be controlled by signal variations now in use for television transmission. This may be effected by providing a series of circuits of the type described above, each including a crystal which resonates at a different but nearby frequency and having the impedances of thecircuit so related that they operate as filter circuits to respectively concentrate practically all of the energy in a given frequency range in the proper crystal, and connecting these circuits in parallel with each other; whereby there is provided a' combination which responds to driving potentials which extend over a wide frequency range, such as, for example, of the order of a million cycles or more. A detaileddescription of the invention follows and is illustrated in the attached drawing in which:

Fig. 1 is a diagrammatic representation of an electro-optical system embodying the invention;

Fig. 2 diagrammatically illustrates a system comprising a plurality of piezoelectric crystal light valves controlled by a signal current including variations extending over an extremely wide frequency band; and

Fig. 3 is a perspective view of a crystal structure which may be used in either of the arrangements illustrated in the preceding figures.

Referring to Fig. 1 there is shown, by way of example, a signal recording system comprising a source of chromatic light i, a lens 2, a polarizer 3,

a mask 4 having an aperture 5, a vibrating crystal 6 axially aligned with the aperture 5, an analyzer I, a lens 8, an apertured screen 9, and a light I plied by lens 8. The crystal 6 is provided with a pair of electrodes i2 supplied from a source of signal modulated carrier current l3 over a circuit I l, including. an amplifier 39 provided with a tuned input circuit, an adjustable resistance 55 and an inductance it connected in series with the crystal electrodes, and an adjustable condenser lll connected in shunt to these electrodes.

Light radiated from the source i is focused upon the recording film Ill through the aperture in screen 9 by lenses 2 and 8 after passing through the polarizer 3, the analyzer l and the transparent crystal 5. The effect of the signal modulated carrier current, supplied over the circuit i4, is to cause the crystal to vibrate at an amplitude proportional to the voltage of the impressed current. During such vibration, the plane of the polarized light received from the. polarizer 3 is rotated sothat an amount'of light proportional to the amplitude of the vibration of the crystal 6 passes through the analyzer I to control a record on the film It.

The Nicol prisms, i. e., the polarizer 3 analyzer l, and the electrically energized crystal 6 function as a shutter or light valve tocontrol the passage of light from the light source I to the light record film or blank it. The capacity of this shutter or valve to pass light therethrough is dependent on the electro-activity of the crystal.

Light valves or shutters of the type described above have been disclosed in the prior art. However, they depend for their operation on the resonant response of the crystal per se. As is well known, the characteristic impedance of a crystal, for applied driving potentials, is such that its response is a maximum when a driving potential of anti-resonant frequency is applied to it, but the response drops to an extremely low value, when the frequency of the applied potential diifers from that of the anti-resonant frequency of the crystal by an extremely small amount. As a matter of fact, the response of the crystal will only be substantially uniform, provided the frequency of the applied driving potential does not exceed a small fraction of one per cent of the anti-resonant frequency of the crystal.

and

voltage required to produce a given amount of strain in the crystal is a minimum at this frequency and increases rapidly on either side thereof. By properly adjusting the value of re sistance I5, the magnitude of the voltages applied to the crystal may be so regulated that the amplitude of the response of the crystal may be made equal at the three frequencies under consideration and hence the crystal will be deformed substantially uniformly over the frequency range identified by the limiting frequencies, and hence the variation of the plane of polarization of light passing through the crystal will be substantially the same and the amount of light transmitted therethrough will be substantially constant.

It can be shown that the maximum band width which may be obtained is directly proportional to the coefiicient ofcoupling between the electrical and mechanical systems. This may be expressed by the equation,

in which is and ii are the maximum and mini mum limiting frequencies, respectively and I2 is the anti-resonant frequency. The left-hand member of the equation therefore defines the ratio of the band width to the anti-resonant frequency and K is the coefficient of coupling.

K may also be expressed in terms of the constants of the equivalent electrical circuit of the crystal as follows:

where C3=the series capacity of the equivalent crystal circuit, and

C4=the parallel capacity of the equivalent crystal circuit.

upon the piezoelectric constants of the quartz and other factors which are not readily determinable. For this reason, it is more convenien to express K by the equation I where f =the anti-resonant frequency of the crystal,

and fa=the resonant frequency of the crystal.

These two frequencies may be readily determined by measuring the current through the crystal as the frequency of the applied potential is varied and noting the maximum and minimum current values.

From Equation 1, it will be evident that the 'coefiicient of coupling, and hence the band width, is aimaximum when the series capacity is a maximum. In order to secure a maximum series capacity, the crystal 6 is provided with electrodes I2 as shown in Fig. 3. These electrodes are applied to the large faces of the crystal, 1. e., to: the surfaces perpendicular to the electrical axis, by depositing a layer of silver, or 1 other conductive metal, to secure an intimate contact over the whole surface, whereby the gap between these electrodes and the crystal is reduced to a minimum, and contact is made with the respective electrodes by point or knife edge contacts 25 and 26 which clamp the crystal at a. nodal point on its optical axis. The electrode coating may be applied by electrical deposition, or in any other manner adapted to insure an intimate contact with the crystal.

- In the operation of the crystal for the purposes of this invention, it is important that the crystal be free from secondary modes of vibration over the transmission band of uniform response, as secondary vibrations seriously alter the transmission characteristics of the crystal and may cause it to shatter. In general the number of secondary frequencies increases as the mode of higher frequencies is utilized. For this reason, low frequency modes of vibration are used and, as the third harmonic longitudinal vibration along the length, or Y axis, was found to be free from secondary frequencies when the ratio of the dimensions of the crystal arekept within limits the coatings applied to the respective crystal surfaces were each divided into three sections and the alternate sections were connected together as shown in Fig. 3, in order to obtain maximum coupling for the third har- .monic vibration.

According to Fig. 3, the electrode coating l2 on one side of the crystal is divided into three sections I2a, l2b, and He, and the electrode coating on the other surface of the crystal is divided into three corresponding sections 1211, l2e

and I2/, the sections 12a, [2e and I20 being con-' nected together by connectors 2| and 22. The former connector extends across one end and along part of the opposite side of the crystal to I2e, while 22 extends from |2c along the-remainder of this side of the crystal and across its other end to I20. Sections I201, I21) and 12) are connected together by connectors 23 and 24. The former extends across the same end of the crystal as 2! and along its other side to l2b, while 24 extends along the same side of the crystal as 23 and across the same end of the crystal as'22 to the section l2). Electrical contact is made with the three sections l2a, l2e, I20 and l2d, l2b and I2), respectively, and the crystal is supported, by point or knife edge contacts 25 and 26, which clamp the crystal at a nodal point on the length, or Yaxis, of the crystal. Light from ed as illustrated in Fig. 3. This crystal was supplied with alternating current of 162 kilocycles to cause the crystal to vibrate in its third harmonic mode along its length, or Y axis, over a circuit which was not provided with a shunting condenser I! but included 'a resistance 25 of 8800 ohms in series with an inductance 86, which was adjustable to vary the selectivity of the complete circuit, including these impedances and the crystal, over-the frequency range between 150 kilocycles to approximately 170 kiloc-ycles. As the selectivity of the circuit was varied, the light transmitted through the crystal, as measured by a Macbeth illuminometer, was substantially constant over a frequency range of 9500 cycles, the resonant frequency of the crystal being at substantially the mid-point of this range, whereas when the same crystal was similarly energized over a circuit which did not include the impedances l5 and I6, the light transmitted through "it was substantially constantover a range 'of 42 cycles, the resonant frequency of the crystal being approximately the mid frequency of this range.

Similar results were obtained with other crystals I having somewhat different anti-resonant frequencies, respectively, than that supplied to the crystal referred to above. In general, it was found necessary to adjust the value of the inductance so that the upper limiting frequency of the band, on which a uniform response was obvtained, was closer to the anti-resonant frequency of the crystal than the lower limiting frequency of this band. However, by tuning the input of the amplifier toward the upper frequency of the voltage supplied to the crystal circuit, the voltages applied to the'crystal in the upper portion of the frequency range were increased with respect to those in the lower portion of this range. In this manner the band over which uniform response occurred was adjusted so that the antiresonant. frequency was approximately equal to the mean of the limiting frequencies. Under different operating conditions, it maybe that the anti-resonant frequency is closer to the lower limiting frequency of the uniform response band than to the upper limiting frequency, in which event, the tuningofthe amplifier inputcircuit would be adjusted toward the lower limiting frequency of the. voltage suppliedto the crystal circuit. I

In certain cases it may not be necessary to obtain a uniform response band as wide as that provided by the circuit described above. In this case, the sensitivity of the light valve can be increased by reducing the band width. This may -be accomplished by adjusting the condenser l1.

Itcan readilybe shown, that, up to the time that dissipation plays a controlling part, the sensitivity of the crystal will be inversely as the square of the band width. Hence, if the band width is decreased by one-half by using a paralleling condenser, the sensitivity will be increased four times and the amount of power to 1 obtain the same sensitivity will be decreased sixteen times. However, as the band width is decreased, the effect of dissipation becomes more prominent and the gain in sensitivity obtained, by decreasing the band width, becomes less.

' A crystal valve of the type described above may be used in a sound recording system or a picture transmission system, andby utilizinga crystal having an anti-resonant frequency of the order for use. in a television system designed to gencrate an image current extending over a limited frequency range, for example, of the order of 50 kilocycles. i

For high quality television systems in which the image band may extend to upwards of a million or more cycles, a light valve of the type disclosed in Fig. 2 may be used.

The valve of Fig. 2 includes a series of crystals 3i, 32, 33 and 3 1, each included in a respective filter circuit 35, ea, 3? and 38. By designing the crystal circuits as described above and so that they are respectively effective to select contiguous portions of a continuous frequency band and by connecting a plurality of these circuits in parallel with respect to the signal supply source It, there is provided a light valve assembly having a substantially uniform response over a Wide band of frequencies.

Noloss of sensitivity results from this method of widening the frequency band, because practically all of the energy in a given frequency range is selected and applied to the proper crystal by the impedance relation of the parallel filters. Since filters of this type have a high impedance outside the transmission band which corresponds to the radiation band of the crystal used as a unit of the light valve assembly, they may be connected in parallel and practically all of the energy lying in a given frequency range will be delivered to the filter having a band-pass in that range.

If, for example, eight crystal circuits are used,v

it is possible to provide a light valve having a substantially uniform response over'a frequency range the upper limiting frequency of which is twice that of the lower limiting frequency.

By. including an amplifier having an adjustable tuned input circuit, as shown in Fig. 1 in each of circuits 35, 35, 3'5 and 38, it ispossible to adjust the position of the frequency band over which the response of each'circuit is uniform with respect to the anti-resonant frequency of the crystal included in the circuit.

While in the preceding description certain details and numerical examples are set forth for the purpose of disclosing the, principles of the invention, it will be apparent that the dimensions of the by increasing the coupling coefiicient the uniform,

response band width for a given crystal may be increased, and byplaclng the crystal in'a vacuum, the effect of arcing can be eliminated and hence a large percentage of the available light can be usefully employed. p

, While the use of monochromatic light appears preferable, various optical arrangements using white light have been tested with some measure of success, from which it appears that, with some refinements, it is possible to provide a light valve of the type herein disclosed to control white light.

What is claimed is:

LA lightmolulating apparatus comprising a piezoelectric element included in a circuit supplied withsignals extending over a frequency band corresponding to at least one per cent of the resonant frequency of said element, and a plurality of impedances included in said circuit for causing the response of said element tofbe substantially uniform over said band. i f

2; A light modulating apparatus comprising a piezoelectric element which responds uniformly to a frequency band definitely related to its resonant frequency, and means for extending the limits of said uniform response band comprising a plurality of impedances connected in series with said element.

3. A light modulating apparatus comprising a piezoelcctric'elernent which responds substantially uniformly to signal potentials extendingover a frequency band definitely related to its resonant frequency, a plurality of impedances connectedin series with said. element for widening the band to whichits response is uniform, and an impedance in shunt to said element for controlling the extent of said band.

4. An electro-optical system including a source supplying light of constant intensity, a plurality of parallel connected circuits respectively selective of a different frequency range, each circuit comprising a piezoelectric crystal in the path of said light and a plurality of impedances in series with said crystal. v

5. A light valve comprising a piezoelectric crystal, means for causing its response to be substantially uniform over a wide frequency band of driving potentials, andmeans for adjusting the position of said frequency band with respect to included therein for rendering the respective crystals substantially uniformly responsive to driving potentials extending over a frequency range corresponding to at least one per cent of the resonant frequency of the crystal and included in adjacent portions'of a wide frequency band.

8..A lightvalvc coniprisinga plurality of circuits; a piezoelectric element and aplurality of impedances included in each said circuit for causing said element to be uniformly responsive to driving potentials in a wide frequency band, and means supplying driving potentials extending over a frequency range including a plurality of said bands in parallel to said circiuts.

9. A light valve which substantially'uniforrnly responsive to a signal extending over a. Wide frequency range comprising a source of signals, a plurality of filter circuits connected in parallel'to said source, said circuits respectively including a piezoelectric crystal and'means for causing said crystal'to be uniformly responsive to signals included in a wide frequency band, said circuits being selective of. contiguousbands with insaid range. 1 i

10. An electro-optical system including a source supplying light of constant intensityJ-a' plurality of parallel connected circuits respectively selective of a different frequency range, each circuit comprising a piesoel'ectric crystalv in the path of said light and a plurality of iinpedanees for controlling a characteristic of saidcrystal.

11. An electro-optical system including a source supplying light of constant intensity, a plurality ofparallelconnected circuits respectively selective of a different frequency range, each circuit comprising a piezoelectric crystal in the path of said light, a plurality of impedances for controlling a characteristic of said crystal, and means for modifying said characteristic.

12. An electro-optical system including a source supplying light of constant intensity, a plurality of parallel connected circuits respectively selective of a different frequency range, each circuit comprising a piezoelectric crystal in the path of said light, means for causing the response of said crystal to be substantially uniform over a Wide frequency band of driving potentials, and means for controlling said potentials.

13. A light valve comprising a piezoelectric crystal, means for causing its response to be substantially uniform over a Wide frequency band of driving potentials, means for adjusting the position of said frequency band with respect to the resonant frequency of the crystal, and means for controlling the width of said frequency band.

14. A light modulating apparatus comprising a piezoelectric element included in a circuit supplied with signals extending over a frequency band corresponding to at least one per cent of the resonant frequency of said element, and a plurality of impedances included in said circuit, said impedances comprising means for causing the response of said element to be substantially uniform over said band and means for controlling the Width of said band.

15. A light modulating apparatus comprising a piezoelectric element included in a circuit supplied with signals extending over a frequency band corresponding to at least one per cent of the resonant frequency of said element, a plurality of impedances included in said circuit for causing the response of said element to be substantially uniform over said band and means for adjusting the position of said band With respect to the resonant frequency of said element.

16. A light modulating apparatus comprising a piezoelectric element included in a circuit sup-v plied with signals extending overa frequency band corresponding to at least one per cent of the resonant frequency of said element, a plurality of impedances included in said circuit, said impedances comprising means for causing the response of said element to be substantially uniform over said band and means for controlling the Width of said band, and means for adjusting the position of said band with respect to the resonant frequency of said element.

1'7. A light modulating apparatus comprising a piezoelectric element which responds uniformly to a frequency band definitely related to its resonant frequency, means for extending the limits of said uniform response band comprising a plurality of impedances connected in series with said element, and means for adjusting the position of said uniform response band With respect to the resonant frequency of said element.

18. A light modulating apparatus comprising a piezoelectric element which responds uniformly to a frequency band definitely related to its resonant frequency, means for extending the limits of said uniform response band comprising a plurality of impedances connected in series with said elecontrolling the extent of said uniform response band.

WARREN P. MASON. 

